Over temperature and under temperature detection are important for the protection of personnel, equipment, and agricultural crops. Electronic equipment will fail if allowed to exceed specified maximum and minimum temperatures. Incipient mechanical equipment failure can be detected by sensing overheating and shutting down the equipment. Ammunition dumps, power cables, coal conveyors, etc. must all be operated below maximum temperature levels to avoid damage. Agricultural crops must be kept from freezing to survive. Certain chemical processes and fluid transport pipes must be kept at temperatures well above ambient to avoid adverse chemical reactions or increased viscosity. These are but a few examples of extended or spatially distributed systems where knowledge of undesirable temperature excursions is essential.
Heating and cooling systems are often provided to avoid undesirable temperature excursions. Such systems, however, are generally energy intensive, and, with the advent of the energy crisis, expensive to operate. Furthermore, heating and cooling systems are often operated when they are not needed due to the lack of over- or under-temperature information. Finally, such systems often fail without warning and without any indication to the user, resulting in undesirable temperature excursions which often lead to catastrophic failures and safety hazards.
Temperature excursions are conventionally measured by a number of devices which change a physical property with temperature. Perhaps the simplest device is the thermostatic switch or thermostat which either opens or closes an electrical circuit when the temperature rises above or falls below a predetermined value. The thermostat is a discrete device but has been used to monitor extended or spatially distributed systems for temperature excursions through the use of multiplexing or by running individual wires to each of the thermostats. Multiplexing requires an electronic circuit and electrical power at the location of each thermostat to encode the information for transmission. The use of individual wires produces a heavy, inflexible, expensive, and generally impractical cable when large numbers of thermostats are required.
The instant invention improves over the above described techniques by providing a light, flexible cable containing only two conductors and thermostatic devices distributed along the length of the cable, encased within an environmentally resistant jacket. Multiplexing circuits and individual wires are no longer necessary to detect and locate a temperature excursion.
The above discussion was confined to cables containing discrete temperature sensitive devices. There also exists another class of linear temperature sensors which does not utilize discrete devices but rather utilizes continuously distributed temperature sensitive material. The major advantage of the latter class is the ability of the sensor to detect an abnormal temperature excursion at any location along its length, as opposed to discrete locations.
There are at least five types of continuously distributed over temperature sensors including: meltable plastics, thermistors, eutectic salts, pneumatics and conductive polymeric compositions. Meltable plastic sensors utilize a twisted pair of wires, each wire encased in meltable plastic material. When the temperature exceeds the material's melting point, the wires touch. To reuse the sensor, the shorted section must be cut out and replaced. A continuous thermistor comprises a continuous negative temperature co-efficient of resistivity material. As the temperature of a sensor made from such material rises, the resistance between two conductors falls. Once the temperature falls, the resistance returns to its original high value. Eutectic salt type sensors utilize a salt compound between two conductors. At the eutectic temperature, the compound melts and its resistance falls connecting the conductors. When the temperature falls, the compound solidifies and the resistance returns to its original high value. In pneumatic devices a tube is pressurized with an inert gas. The application of heat causes an increase in pressure which operates a diaphragm that closes an electrical contact. Upon removing the heat, the pressure falls and the contact reopens. Continuous conductive polymeric composition devices are disclosed in my earlier U.S. patent application Ser. Nos. 88,344, 134,354 (both now abandoned) and 184,647 wherein a conductive polymeric composition is used as a conductor which becomes electrically discontinuous upon exposure to over-temperature.
In exchange for the ability to detect an over-temperature excursion at any location along its length, the principles utilized in the above mentioned devices place fundamental limitations on the construction and performance of distributed sensors which do not exist in the instant invention. The meltable plastic sensor can only be used once and is therefore unsuitable as a monitor where over-temperatures frequently occur, or where the sensor is not easily accessible. The thermistor, eutectic salt, and pneumatic sensors are all metal encased and therefore relatively inflexible and subject to corrosion. In addition, the resistance change is not sharp for the thermistor material and the pressure change is gradual for the pneumatic device making these sensors relatively imprecise. In these devices and in the conductive polymeric composition devices, the number of sensing temperatures is limited by available materials and generally only one sensing temperature for the entire sensor length is provided. In the instant invention discrete thermostats may be selected for any particular temperature, said thermostats being capable of detecting either rise or fall of temperature and therefore capable of detecting both over and under temperature excursions. In conductive polymer type devices the operational length of the device is to some degree limited by relatively low polymer conductivity whereas in the instant invention higher conductivity materials such as copper may be used to interconnect discrete devices over tremendous lengths. Finally, high accuracy is achieved in over or under temperature sensing by the ability to use individual thermostats, ie., switches, which are triggered at precise temperatures, provide huge resistance changes, and have extremely stable trip temperatures over long periods of time.
One type of continuously distributed under temperature sensor is described in U.S. Pat. No. 4,041,771. This patent describes a sensor consisting of an elongated container which holds a body of material which changes phases at specific temperatures. Specifically, the patent discloses a tube of liquid which changes conductivity upon freezing and thereby changes the capacitance of the sensor. The use of a liquid to solid phase-changing material limits such a device to only the structure disclosed, namely a liquid within a dielectric tube surrounded by a conductor.
In exchange for the ability to detect an under temperature excursion at any location along its length, the use of a liquid to solid transition places fundamental limitations on the construction and performance of sensors which do not exist in the instant invention. First, the use of liquids in tubes limits the flexibility, ruggedness, and durability of the sensor. Second, placing the thermally sensitive material (liquid) on the inside of a co-axial sensor slows the response of the sensor to temperature changes. Third, the magnitude of liquid to solid resistance changes is far less than for electrical switches such as thermostats. For example, for the pure water (18.degree. C.) to ice (-4.degree. C.) transition, the resistance change is only one order of magnitude. (See "International Critical Tables of Numerical Data, Physics, Chemistry and Technology", Volume VI, Mc-Graw-Hill Book Company, Inc., 1929, page 152). This makes precise fault location practically impossible even in short sensor lengths. Fourth, the number of sensing temperatures is limited by available liquid to solid transitions. Fifth, use of insoluble mixtures to achieve transition temperatures unavailable with true solutions is impractical because of the inability to maintain solution homogeneity over the life of the sensor.
The instant invention provides for the use of numerous thermostatic devices without attendant multiplexing circuitry or large numbers of wires so that the sensor cable need not contain large, insensitive intervals. Indeed, these intervals can be made arbitrarily small, the spacing being dictated primarily by economic trade-offs. The result is, for all practical purposes, a sensor cable with none of the disadvantages listed above for continuous sensors, but with a large number of advantages. First, a single cable can serve as both an under and over temperature sensor and locator at different points along its length by choosing thermostatic devices which open on increasing temperature for over temperature sensing and open on decreasing temperature for under temperature sensing. Second, one cable can have various temperature trip points along its length through the use of thermostatic devices with different temperature trip points. Third, innumerable temperature trip points exist with commercially available thermostatic devices. (Sunstrand Data Control catalog is illustrative, showing thermostats available with trip points in the range of -60.degree. C. to 315.degree. C.). Fourth, thermostatic devices exhibit a huge resistance increase upon opening which when combined with the capacitance measuring technique, makes possible location of the temperature excursion with great accuracy. Fifth, thermostatic devices, upon opening, do so without a gradual increase in resistance which makes possible unambiguous capacitance readings and reliable operation.